1. Field of the Invention
The present invention generally relates to an optical lens element and, more particularly, to a complex lens of one-piece construction integrated with a diffraction grating. The present invention also relates to an optical head for accomplishing tracking control on a three-beam scheme which utilizes the one-piece lens integrated with the diffraction grating.
2. Description of the Prior Art
An optical head in an optical information recording and/or reproducing apparatus operates to selectively write or read information on or from an information recording medium. During the operation of the optical information recording and/or reproducing apparatus, a beam of light must necessarily be focused on the track on the information recording medium accurately and, for this purpose, an objective lens in the optical head is controlled to accomplish the tracking control.
In the prior art optical head, a diffraction grating is employed to separate a diverging beam of light from a light source into a zero-order diffracted ray of light (main beam) and two first-order diffracted rays of light (sub-beams). These rays of light are rendered substantially parallel by a collimator lens and are subsequently focused by the objective lens on the information recording medium.
Rays of light reflected from the information recording medium are: after having been rendered parallel by the objective lens, diffracted in a direction different from that along which the beam from the light source travels and towards a light receiving element, through a detection lens system. The light receiving element has a sensor region onto which reflected rays of light of the main beam are received, and two sensor regions onto which reflected rays of light of the respective sub-beams are received. A displacement in tracking can be detected in terms of the difference between the respective intensities of the reflected rays of light of the associated sub-beams, so that a tracking control signal can be obtained to drive the objective lens for compensating for the displacement in tracking. (See, for example, G. Bouwhuis, et al., "Principles of Optical Disc Systems", Chap. 2, published by Adam Hilger, 1985.)
In general, in the event that the diffraction grating is installed having been displaced in position or tilts relative to an optical axis, the prior art optical head poses a problem in that the spacing between the respective light spots formed on the information recording medium by the main beam and the sub-beams deviates to such an extent as to result in an instable tracking control.
Also, while the diffraction gratings generally employed in the prior art optical head have a multiplicity of parallel grooves of a generally rectangular cross-section, it has been observed that, when a beam impinges upon this diffraction grating, not only are the zero- and .+-. first-order diffracted rays of light produced, but .+-. second or higher order diffracted rays of light, which are unnecessary for the optical head, are also produced. This tends to adversely affect the maximized utilization of the rays of light emitted by the light source and, also, since a change in ratio between the intensity of the zero-order diffracted ray and that of the .+-. first-order diffracted rays is sensitive to the depth of each groove in the diffraction grating, even a slight deviation in the depth of the grating grooves during the manufacture of the diffraction grating results in a variation in ratio between the intensity of the zero-order diffracted ray and that of the .+-. first-order diffracted rays.